1. Field of the Invention
The present invention relates to rotor blades for wind turbines and, more particularly, to the attachment means for securing a rotor blade root to the hub of the rotating shaft of a horizontal axis wind turbine and to the method for attaching it and the method for manufacturing the blade.
2. Description of the Prior Art
Wind turbines have been used for many years to harness the power of the wind as a source of energy. In recent years, with the use of ever larger rotor blades on wind turbines, an operational problem has been recognized which is fatigue failure of the blades at the point of attachment of the blade root to the hub of the rotational shaft of the turbine. Most wind turbine rotors rotate many millions of times per year and thus accumulate a large number of cycles of stresses. In order to avoid the damaging effects of high cycle fatigue on the wind turbine blade root, and its supporting structure, it is necessary to control the stress on the blade/hub connection.
Structural failure due to cyclic stress fatigue of the turbine blade roots is believed to be due to stress loading of dissimilar materials at the attachment point of the blade root. Wind turbine rotor blades are usually very large and made of composite materials such as fiberglass and wood/epoxy while the hub of the rotational shaft of the wind turbine is usually constructed of steel. These rotor blades are considerably different than an aircraft propeller which is a complicated high rotational speed wind pushing mechanical device with internal mechanisms for changing the pitch of the blade in response to operator control. A wind turbine rotor blade on the other hand is a very long slow rotational speed blade pushed by the wind to harness the wind energy. There are very few common features between propellers and wind turbine blades in their construction except for a general similarity in appearance in some designs.
In operation, the bending loads which are imposed on a turbine blade by gravity, wind, and other forces are translated at the blade root/hub interface into tension, compression, and shear loads. By far the largest loads are those occurring in tension and compression when the individual blade is horizontal and wind loaded.
Various devices and methods have been used in the prior art to attach rotor blades to the rotational shafts of wind turbines. Most of these embody the use of a metal or combination metal and composite material flange on the root of the blade that is bolted to the hub of the rotational shaft by steel rods or bolts, or they insert steel bolts or rods into the composite blade structure.
FIG. 1 of the drawings shows one example of a blade attachment means where the filament-wound spar and skin laminant of the root of the blade are bound around a steel root compression member and held thereagainst by an overlay steel clamp plate.
FIG. 2 shows a different steel root connection having the filament-wound spar laminant of the root of the blade secured internally in the connection in a cylindrical male-female relation. Compression bolts and adhesive hold the laminant in the steel foot connector which in turn is bolted to the rotation hub of the wind turbine.
FIG. 3 shows a rotor blade root laminant structure of woodply and epoxy which has steel attachment stud rods secured internally of the blade root by an epoxy adhesive and which project from the bottom of the blade root. The stud rods are in turn bolted to the steel hub of the rotation shaft of the turbine.
FIG. 4 shows yet another blade root connector utilizing a compression ring flange which is bolted to the hub of the turbine. The laminant rovings of the rotor blade root are flared around the compression ring flange and held thereto by a washer plate. The washer plates are held in place by bolts which project through the whole sandwich assembly: the washers, the laminant, and the compression ring flange to hold the blade root to the hub of the rotational shaft of the turbine.
These methods of attachment all experience and are subject to the major problem of cyclic fatigue stress. In methods which employ inserting the laminated composite blade structure into a metal flange or connector, examples of which are shown in FIGS. 1, 2, and 4, the interface are creates massive stress risers in the composite material at the metal hub interface which can lead to fatigue failures as these joints are experiencing alternating cycles of tension and compression.
In designs utilizing a flange of composite laminate and then clamping that composite flange to the rotational hub, such as are illustrated by the example of FIG. 3, the loads on the blade are converted into alternating bending stresses in the composite hub. These materials which are well-constructed for tension and compression loads are not well-suited for bending loads and also fail in fatigue. The worse possible loading cycle, that of alternating compression and tension, as well as bending, is also present at this bend in the composite material.
The third method which has been used to attach the blade to the shaft, as shown in FIG. 3, which involves embedding a steel rod into the composite material and then using this rod to take all the compression and tension loads, also has problems in fatigue; primarily because of the sharply different physical characteristics of the two materials in the presence of alternating compression and tension loads. Their rates of thermal expansion are radically different as are their modulus in compression and tension. Over time, with tens of millions of cycles of fatigue loading, these differences cause problems with the adhesive bonds. Also, bending and flexing of the blade under load causes stress in the composite shell itself where these rods are embedded which also can lead to fatigue failure.
All of the above methods are used to attach blades to the hubs of rotational shafts of wind turbines and each of them provides some period of useful life. However, all of them fail at some point measured in tens of millions of cycles. Thus, there is a need for a rotor blade root connection method which will last for hundreds of millions of cycles.